Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability, and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours & Company under the trademark "Surlyn.RTM." and more recently, by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademarks "Escor.RTM." and the tradename "Iotek", have become the materials of choice for the construction of golf ball covers over the traditional "balata" (trans polyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. In some instances, an additional softening comonomer such as an acrylate can also be included to form a terpolymer. The pendent ionic groups in the ionomeric resins interact to form ion-rich aggregates contained in a non-polar polymer matrix. The metal ions, such as sodium, zinc, magnesium, lithium, potassium, calcium, etc. are used to neutralize some portion of the acid groups in the copolymer resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e. improved durability, etc. for golf ball construction over balata.
Broadly, the ionic copolymers comprise one or more alpha-olefins and from about 9 to about 20 weight percent of alpha, beta-ethylenically unsaturated mono- or dicarboxylic acid, the basic copolymer neutralized with metal ions to the extent desired. Usually, at least 20% of the carboxylic acid groups of the copolymer are neutralized by the metal ions (such as sodium, potassium, zinc, calcium, magnesium, and the like) and exist in the ionic state.
Suitable olefins for use in preparing the ionomeric resins include ethylene, propylene, butene-1, hexene-1, and the like. Unsaturated carboxylic acids include acrylic, methacrylic, ethacrylic, .alpha.-chloroacrylic, crotonic, maleic, fumaric, itaconic acids, and the like. The ionomeric resins utilized in the golf ball industry are generally copolymers of ethylene with acrylic (i.e. Escor.RTM.) and/or methacrylic (i.e. Surlyn.RTM.) acid. In addition, two or more types of ionomeric resins may be blended into the cover compositions in order to produce the desired properties of the resulting golf balls.
Along this line, the properties of the cover compositions and/or the ionomeric resins utilized in the golf ball industry vary according to the type and amount of the metal cation, the molecular weight, the composition of the base resin (i.e. the nature and the relative content of the olefin, the unsaturated carboxylic acid groups, etc.), the amount of acid, the degree of neutralization and whether additional ingredients such as reinforcement agents or additives are utilized. Consequently, the properties of the ionomer resins can be controlled and varied in order to produce golf balls having different playing characteristics, such as differences in hardness, playability (i.e. spin, feel, click, etc.), durability (i.e. impact and/or cut resistance), and resilience (i.e. coefficient of restitution).
However, while there are currently more than fifty commercial grades of ionomers available from DuPont and Exxon with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e. relative content of ethylene and methacrylic and/or acrylic acid groups), the degree of neutralization and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop golf ball cover compositions exhibiting not only the playability characteristics previously associated with the balata cover, but also the improved impact resistance and carrying distance properties produced by the ionomeric resins. Thus, an object of the present invention is to provide golf ball cover compositions which, when utilized in golf ball construction, produce balls exhibiting improved travel distance while maintaining satisfactory playability and durability properties.
In enhancing the distance a golf ball will travel when hit, there are a variety of factors which are considered. The coefficient of restitution, along with ball size, weight and additional factors such as club head speed, angle of trajectory, and ball aerodynamics (i.e., dimple pattern), generally determine the distance a ball will travel when hit. Since club head speed and the angle of trajectory are not factors easily controllable, particularly by golf ball manufacturers, the factors of concern among manufacturers are the coefficient of restitution and the surface dimple pattern of the ball.
A golf ball's coefficient of restitution (C.O.R.) is the ratio of the relative velocity of the ball after direct impact to that before impact. One way to measure the coefficient of restitution is to propel a ball at a given speed against a hard massive surface, and measure its incoming velocity and outgoing velocity. The coefficient of restitution is defined as the ratio of the outgoing velocity to incoming velocity of a rebounding ball and is expressed as a decimal. As a result, the coefficient of restitution can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
The coefficient of restitution of a one-piece golf ball is a function of the ball's composition. In a two-piece or a multi-layered golf ball, the coefficient of restitution is a function of the core, the cover and any additional layer. While there are no United States Golf Association (U.S.G.A.) limitations on the coefficient of restitution values of a golf ball, the U.S.G.A. requires that the golf ball cannot exceed an initial velocity of 255 feet/second. As a result, golf ball manufacturers generally seek to maximize the coefficient of restitution of a ball without violating the velocity limitation.
In various attempts to produce a high coefficient of restitution golf ball exhibiting the enhanced travel distance desired, the golfing industry has blended various ionomeric resins. However, many of these blends do not exhibit the durability and playability characteristics necessary for repetitive play and/or the enhanced travel distance desired.
The present invention is directed to the preparation of new cation neutralized ionomer resins containing relative high amounts of acid (i.e. greater than 16 weight percent acid, preferably from about 17 to about 25 weight percent acid, and more preferably from about 18.5 to about 21.5 weight percent acid) and partially neutralized with sodium, manganese, lithium, potassium, zinc, magnesium calcium and nickel ions. The new cation neutralized high acid ionomers produce, when blended and melt processed according to the parameters set forth below, cover compositions exhibiting enhanced coefficient of restitution values when compared to low acid ionomers, or blends of low acid ionomer resins containing 16 weight percent acid or less. The new high acid ionomer cover compositions produce golf balls which exhibit properties of enhanced carrying distance (i.e. possess higher coefficient of restitution values) over known ionomer blends such as those set forth in U.S. Pat. Nos. 4,884,814 and 4,911,451, without sacrificing desirable characteristics such as playability and/or durability.
Along this line, until relatively recently, all of the ionomer resins commercially available contained at most 15 to 16 weight percent carboxylic acid. In 1989, DuPont introduced a number of new high acid ionomers and suggested that these new ionomers may have some use in previously known low acid ionomer applications such as the production of shoe soles, box toes, bowling pins, golf balls, ski boots, auto trim, etc.
Furthermore, DuPont suggested in a research disclosure (E. I. DuPont de Nemours & Co., Research Disclosure No. 297,003) that ionomers produced from polymers of ethylene acrylic acid or methacrylic acid containing greater than 15 weight percent acid can be melt processed to produce articles (i.e. golf balls, foot wear, ski boots, cosmetic bottle cap closures and so on) with good properties (i.e. improved stiffness, hardness and clarity) when compared with ionomers with lower acid levels.
However, not only has little information been provided concerning the acid levels and types of effective ionomers, particularly with respect to the art of golf ball manufacturing, it has been found that many cover compositions produced from polymers of ethylene/acrylic acid or ethylene/methacrylic acid containing greater than 15 weight percent acid have been dissatisfactory in that these compositions exhibit processing problems or are generally short on distance and/or durability and thus, are not particularly commercially viable. Similar poor results have been produced with covers composed of blends of high and low acid ethylene/acrylic acid or ethylene/methacrylic acid polymers and/or covers produced from single high acid ionomers.
However, notwithstanding the above difficulties, it has been discovered that improved golf ball covers can be produced from specific blends of high acid ionomers (i.e. ionomer resins containing greater than 16 weight percent acid, preferably from about 17 to about 25 weight percent acid, and more preferably from about 18.5 to about 21.5 weight percent acid) which do not exhibit the processing, distance and/or durability limitations demonstrated by the prior art.
In this regard, it has been found that blends of specific high acid ionomer resins, particularly blends of sodium and zinc high acid ionomers, as well as blends of sodium and magnesium high acid ionomers, extend, when utilized in golf ball cover construction, the range of hardness beyond that previously obtainable while maintaining the beneficial properties (i.e. durability, click, feel, etc.) of the softer low acid ionomers disclosed in U.S. Pat. Nos. 4,884,814 and 4,911,451. These blends produce harder, stiffer golf balls having higher C.O.R.s, and thus longer distance. This discovery is the subject matter of U.S. application Ser. No. 776,803, filed on Oct. 15, 1991, and currently copending herewith.
The present invention is directed to the development of a number of new high acid ionomers, particularly new metal cation neutralized acrylic acid based high acid ionomer resins, which exhibit, when utilized for golf ball cover construction, cover compositions having further improved hardness and resilience (C.O.R.) properties. The new metal cation neutralized acrylic acid based high acid ionomer resins, as well as specific blends of these resins, are particularly valuable in the field of golf ball production.
Furthermore, as a result of the development of a number of new acrylic acid based high acid ionomers neutralized to various extents by several different types of metal cations, such as by manganese, lithium, potassium, calcium and nickel cations, several new high acid ionomers and/or high acid ionomer blends besides sodium, zinc and magnesium high acid ionomers or ionomer blends are now available for golf ball cover production. It has been found that many of these new cation neutralized high acid ionomer blends produce cover compositions exhibiting enhanced resilience (i.e. longer distance) due to synergies which occur during processing. Consequently, the new metal cation neutralized acrylic acid based high acid ionomer resins of the present invention may be blended to produce substantially harder golf balls having higher C.O.R.'s than those produced by the low acid ionomer covers presently commercially available.
These and other objects and features of the invention will be apparent from the following description and from the claims.